This invention relates to a speech synthesis system having circuit components capable of being implemented as an integrated circuit device utilizing complementary metal-insulator-semiconductor technology to enable the system to operate at a relatively low voltage. More specifically, this invention relates to a low voltage digital speech synthesis system with circuit components implemented in a complementary metal-insulator-semiconductor integrated circuit device in which the digital-to-analog converter is of a pulse width modulated type offering accurate conversion of digital speech signals into analog signals from which synthesized human speech may be generated even though the low voltage operation restricts the extent of the voltage swings normally required in converting digital signals to analog signals.
Several techniques are known in the prior art for digitizing human speech. For example, pulse code modulation, differential pulse code modulation, adaptive predictive coding, delta modulation, channel vocoders, cepstrum vocoders, formant vocoders, voice excited vocoders, and linear predictive coding techniques of speech digitization are known. The techniques are briefly explained in "Voice Signals; Bit by Bit" on pages 28-34 of the October, 1973 issue of IEEE Spectrum.
In certain applications and particularly those in which digitized speech is to be stored in a memory, most researchers tend to use the linear predictive coding technique because it produces a very high quality speech using rather low data rates. An excellent example of the use of linear predictive coding systems, implementable in integrated circuit techniques may be seen in U.S. Patent Application Ser. No. 901,393, filed Apr. 28, 1978, now U.S. Pat. No. 4,209,836 issued June 24, 1980. The speech synthesis system described in the aforementioned U.S. Pat. No. 4,209,836 utilizes frames of data which are comprised of digital representations of pitch, energy and certain linear predictive coefficients for controlling a digital filter. The system described in the aforementioned U.S. Pat. No. 4,209,836 is capable of producing high quality synthetic human speech at a bit rate of as low as 1200 bits per second, utilizing a fixed rate of data frame entry. As integrated speech synthesis systems continue to develop a need arises for low voltage applications. Such low voltage applications which are known in the art wherein a small battery source may provide the electrical supply, have two inherent problems. Firstly, in an embodiment such as an electronic learning aid, translator, or calculator, the current source required for such a system will result in short battery life. Secondly, a low voltage speech synthesis system will have difficulty in accurately simulating the human speech waveform while constrained to a low voltage swing. Standard digital-to-analog converter devices utilized in integrated circuit speech synthesis systems, such as the circuit described in U.S. Pat. No. 4,209,836, lack sufficient accuracy and resolution to be utilized in low voltage applications such as complementary metal-insulator-semiconductor technology. An ideal solution to the aforementioned problem, would require a speech synthesis system capable of synthesizing human speech from frames of data at sufficiently low voltage levels to reduce the current drain which shortens battery life, and a digital-to-analog converter capable of producing accurate representations of complex waveforms at low voltage levels.
It is therefore one object of this invention to improve speech synthesis technology.
It is another object of this invention to provide a speech synthesis system capable of accurately synthesizing human speech at low voltage levels.
It is still another object of this invention to provide a speech synthesis system with a digital-to-analog converter capable of accurately modelling complex waveforms at low voltage levels.
The foregoing objects are achieved as now described. A low voltage speech synthesis system with a linear predictive filter is provided. The linear predictive filter utilizes reflection coefficients to produce digital signals representative of human speech. An integrated circuit controller controls the access of digitized speech data stored in a memory circuit. Synthesis and control circuits are implemented in complementary metal-insulator-semiconductor technology to provide low voltage operation. A pulse width modulated digital-to-analog converter also implemented in complementary metal-insulator-semiconductor technology permits accurate conversion of digital signals into analog signals in low voltage operation.